I just watched a fascinating documentary about being trapped in a car in serious weather conditions. The best advice people who survived flooding and snow storms in their cars gave was to wind your windows down!
Sounds contrary but then the why is more understandable than you might think.
The big problem with being trapped in it car is that the electrics can fail at any given time. With most newer car models being all electric, you can easily find yourself trapped in a car you thought was a safe place to be!
If the electrics go, or the battery dies, then your main means of escape - the windows and sun roof - become shut tight! And before you can muster enough swear words to compensate, you're trapped inside a big metal box.
So in case of flooding- if you find yourself trapped in a car, open the windows (at least 6 inches to give you chance to push them open) so that you can get out alive and well should you need to!
And don't forget a spare pair of Wellington boots just in case.